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Factors that influence erosion and runoff

thinksoils Factors that influence erosion and runoff

Environment Agency thinksoils factors that influence erosion and runoff

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Factors that influence erosion and runoff 8


Soil

Landscape

Weather

Land use



Factors that influence erosion and runoff Soil ● texture ● wetness ● structure ● soil surface roughness

Landscape ● steep slopes ● field size and valley features ● proximity of watercourses ● field tracks and roads

Weather ● rainfall intensity ● climate and soils

Land use ● risks associated with crops and livestock


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soil texture

Soil texture Soil with high sand content is not cohesive and has a high risk of erosion

Soil texture Soil texture refers to the relative proportion of clay, silt and sand.

This restricts movement of water through the soil and increases the risk of runoff.

The risk of runoff and erosion is affected by small differences in texture. This is because texture influences the degree of percolation of water through the soil, and also the stability of soil.

Soils with low clay content are less cohesive and are inherently more unstable. These soils are at greater risk of erosion by water and wind.

Soils containing large proportions of sand have relatively large pores through which water can drain freely. These soils are at less risk of producing runoff. As the proportion of clay increases, the size of the pore space decreases.

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Environment Agency thinksoils factors that influence erosion and runoff / soil texture


soil wetness

Soil wetness

Mottles indicate periods of waterlogging

Soil wetness Naturally waterlogged soils are known as gley soils. Soils affected by a high water table are groundwater gley soils, and those that are waterlogged due to slow percolation of water are known as surface water gley soils. Wet soils have greater risk of runoff. After the summer and in well structured soils, without deep fissures or cracks, rain wets the soil progressively from the surface. This creates a wetting front that moves down the soil profile. Compacted layers within the soil will affect this wetting front and it may cause areas of surface ponding across a field.

Grey and bluish-grey colours develop in saturated soils due to a lack of air and the reduction of iron compounds. A patchwork of bluish-grey colours occur together with orange, yellow or rusty colours (mottles) in the zone where there is waterlogging for part of the year. Orange / yellow colours occur where iron has reoxidised.

Environment Agency thinksoils factors that influence erosion and runoff / soil wetness

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soil structure Good soil structure

Soil structure Poor soil structure

Soil structure Soil structure refers to the arrangement of soil particles in the soil. Clay content, organic matter (and in some soils calcium and iron compounds) help to bind the soil together into structural units, aggregates or peds. Well structured soil allows the free movement of air and water through fissures (or cracks) between the structural units. Pores within the units also allow the movement of air and water. A soil with poor soil structure has a high risk of generating runoff. The risk of runoff is greatest when poor soil structure is near the soil surface. Soil structure deteriorates when structural units are deformed producing a dense single mass of soil (or large soil units). This occurs when pressure is applied to a wet and soft soil.

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Pressure squeezes the soil units together and reduces pore space within the units. A dry soil can withstand pressure without deforming soil structure. Some soils are unstable when clay, calcium or organic matter content is low. Unstable aggregates disperse when wet, forming a solid mass as the soil dries. Where this occurs at the immediate soil surface, the soil may form a cap or crust. Soils can restructure due to natural fracturing processes when clay shrinks and swells, and by cultivation. Biological activity also restructures soil.

Environment Agency thinksoils factors that influence erosion and runoff / soil structure

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soil surface roughness

Soil surface roughness

Ploughed land provides surface storage of rainwater

Soil surface roughness Rough surfaces (e.g. in ploughed land, coarse seedbeds, or pressed land with indentations) help to slow down runoff. Roughness provides storage of rainwater, allowing water to collect before it soaks into the soil. For some fields, extra storage can be created if the ploughed land is worked across a slope and not up and down a slope i.e. the ridge and furrows now act as little dams and storage areas. Rough surfaces also help to reduce wind speed at the immediate soil surface, preventing wind erosion.

Environment Agency thinksoils factors that influence erosion and runoff / soil surface roughness

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generic soil groups Generic soil groups

Generic soils groups

There are a number of methods for grouping soils according to their risk of erosion and runoff. The approach used in this document has been to group soils into 5 generic soil groups:

This approach enables soils to be grouped according to risks associated with:

● ● ● ● ●

● shallow soils on chalk and limestone (that are

● clay content (affecting porosity, risk of runoff,

stability and erodibility),

sandy and light silty soils medium soils heavy soils chalk and limestone soils peaty soils

very stable, freely draining with low risk of runoff), ● peaty soils (with high risk of runoff and erosion in the uplands, and high risk of wind erosion in the lowlands).

Soil texture classification for mineral soils, and for soils with high organic matter 100 0

loamy sand

85 80

70

clay loam

sandy silt loam

% organic matter

%c lay sand

0 100

silty clay

sandy clay

sandy loam

t sil

18 15 10

45

%

clay

55 50

sandy clay loam

Peat

20

80

30

10

100

65

silty clay loam

50

50 1

Loamy peat (LP) or 2 Sandy peat (SP) 35

1

Peaty loam (PL) or 2 Peaty sand (PS)

25

25

20

82

Organic mineral soil

silt loam

10 6

10 Mineral soil

100 50

20

% sand

Heavy soils

0

0

50

Sandy and light silty soils

Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups

100

% clay in the mineral fraction 1 2

Less than 50% sand in the mineral fraction 50% sand or more in the mineral fraction

Medium soils

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35

Peaty soils

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generic soil groups

Generic soils groups / sandy and light silty soils

Sandy and light silty soils These soils have low clay content (<18%) in the topsoil, and include sand, loamy sand, sandy loam, sandy silt loam and silt loam textures.

However where the drainage is impeded by high water table or a slowly permeable subsoil, they are at high risk to structural damage and runoff.

Due to the low clay and organic matter content these soils have low aggregate stability. Soils readily disperse (slake) in water causing internal slumping and capping at the surface.

Where runoff occurs these soils have a high risk of erosion. Fine sandy soils have a high risk of wind erosion.

Where these soils are freely draining, and well structured, they have low risk of runoff.

Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups / sandy and light silty soils

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generic soil groups

Generic soils groups / medium soils Medium soils Medium soils include sandy clay loam, clay loam and silty clay loam textures. They have a clay content between 19 and 35% in the topsoil. The higher clay content produces a greater aggregate stability than lighter soils. However, medium soils with a high content of silt or fine sand are not as stable, and are prone to capping, particularly where the organic matter content is low.

Conversely, where the clay content is high, they are prone to waterlogging and structural damage. Structural damage, or poor drainage, in medium soils can lead to runoff and soil erosion, particularly in areas of high rainfall and on slopes.

Clay content in the subsoil (and depth to the water table) affects the drainage of medium soils. Where clay content is low in the subsoil these soils can be freely draining with low risk to structural damage.

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Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups / medium soils

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generic soil groups

Generic soils groups / heavy soils

Heavy soils These soils have a clay content >35% and include sandy clay, clay and silty clay textures. They are naturally slow draining and lie wet for long periods.

The stability of clay and porosity is dependent on the type of clay and the calcium content. Less stable acidic clays have lower porosity and higher risk of runoff than calcareous clays.

Some clay soils have a naturally well developed soil structure which lessens the incidence of waterlogging.

Heavy, slowly draining soils have a high risk of structural damage and generating runoff, but have a low risk of erosion.

Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups / heavy soils

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generic soil groups 3

Generic soils groups / chalk and limestone soils Shallow chalk and limestone soils These thin soils are less than 30cm deep. They are highly calcareous, often with a medium textured topsoil. The soils have stable aggregates and form a strongly developed soil structure. Topsoil can be lost on these soils, exposing the bedrock, due to soil creeping down slopes. The soils are naturally well drained and accept most winter rainfall with low risk of runoff.

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Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups / chalk and limestone soils


generic soil groups

Generic soils groups / peaty soils

Peaty soils This group includes peaty soils where the organic content of the topsoil is more than 20% organic matter (or 12% organic carbon). They include peaty sand, peaty loam, loamy sand, loamy peat and peat textures. Peaty soils are widespread in the uplands and are also found in lowland bogs and river valleys. The Fens of eastern England and the Lancashire Mosses are very productive agricultural soils. These flat, artificially drained peaty soils have a low risk of runoff.

Peaty and organic soils that lie wet in the uplands have a small capacity to accept winter rainfall, with a consequent high risk of generating runoff and soil erosion. In the lowlands and drier parts of the country, peaty soils are prone to wind erosion because of their low density and loose soil structure. When drained they are vulnerable to oxidation, causing peat ‘wastage’.

Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups / peaty soils

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weather Raindrops can detach and disperse soil particles, washing them into pores, causing sealing of the soil surface

weather / rainfall intensity 1

Rainfall intensity Runoff occurs when rainfall intensity exceeds infiltration rate and the soil becomes saturated at the surface. During winter, soils are often described as being at field capacity. This is the maximum water content held in the soil under free drainage. At field capacity, air is held in macropores and the soil can absorb rainfall until it becomes saturated. Naturally well drained soils rarely become saturated and readily absorb most rainfall. Where the surface loses its porosity, runoff can occur on well drained soils when rainfall is as low as 1mm/hr.

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Environment Agency thinksoils factors that influence erosion and runoff / weather

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Applying pressure to moist soil

Applying pressure to moist soil

Structureless mass of soil

weather / climate and soils

Well structured soil


weather

Climate and soils Soil structure is at risk of being damaged when pressure is applied to a wet soil with a putty-like behaviour (i.e. in a moist plastic state). Trafficking, trampling and working of soils when they are too wet is a major cause of deterioration in soil structure. Eastern England is much drier than the west and there is much greater opportunity for timely landwork without damaging soil structure. The risk of wind erosion is greatest in the drier eastern areas of England.

Environment Agency thinksoils factors that influence erosion and runoff / weather

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landscape Steep slopes

landscape

Steep fields can cause water to run off at a rapid rate. This is particularly the case where water percolation into the soil is slow (on naturally slowly draining soil or where there is poor soil structure, or both). Highest risk fields are those greater than 7O. Fields with gentle slopes less than 3O are at lower risk to rapid runoff and erosion. But water can still run and gather momentum on gentle slopes, particularly where the slope is long and infiltration rate is slow. Where the ground is level, water will tend to pond.

Field size and valley features Large fields with long slopes can accumulate large volumes of water. Valley floors can concentrate water flow causing channel erosion. Wheelings and cultivation marks can also influence the direction of water movement. Wind erosion tends to occur on unsheltered land exposed to strong winds, and in areas where wind is funnelled.

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Environment Agency thinksoils factors that influence erosion and runoff / landscape

Proximity of watercourses Fields adjacent to watercourses are at higher risk of causing water pollution associated with runoff than those where there is no connectivity to watercourses.


landscape

landscape

Field tracks and roads Field tracks and roads provide a route-way for runoff, soil sediment and associated pollutants to enter watercourses. Roads and field tracks can link fields with watercourses that are kilometres apart. Runoff from roads and adjacent land can also wash onto fields causing field runoff and erosion.

Environment Agency thinksoils factors that influence erosion and runoff / landscape

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land use / Late sowing in the autumn

land use / cereals

Winter cereals sown during late October and November can put the land at risk of runoff and erosion on sandy and light silty soils due to lack of crop cover over the winter and because of the high risk of the soil surface becoming capped.

Fine smooth seedbeds Land with a fine and smooth seedbed provides little surface storage capacity, and on sandy and light silty soil is at risk of becoming capped causing runoff and erosion. Fine, dry sandy tilths are vulnerable to wind erosion.

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Environment Agency thinksoils factors that influence erosion and runoff / land use / cereals

Compacted seedbeds Land under winter cereals is at risk to generating runoff where the soil is compacted (e.g. when sowing is carried out on wet soil, or where soil has become compacted during previous land work in the rotation).

land use / cereals

Crops established by shallow cultivation are at risk of runoff if there is poor soil structure near the soil surface.


land use /

Compacted cereal stubble Where cereals are harvested in wet conditions, there is a risk of causing soil compaction and runoff. Compacted tramlines and wheelings are at most risk of runoff, especially when aligned up and down a slope.

Environment Agency thinksoils factors that influence erosion and runoff / land use / cereals

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land use / Grassland management Grassland is at risk to poaching and compaction due to grazing when the soil is too wet. Risks are particularly high during autumn and spring.

Wheel ruts

land use / livestock

Farm vehicles can also cause compaction (e.g. transporting heavy trailer loads of silage in wet spring conditions). Soils can restructure in grassland due to high earthworm activity, the dense network of roots, and drying of the soil causing shrinkage and fissuring of clay.

Poached soil

Out-wintering of stock Out-wintering of stock has a high risk of causing soil compaction and runoff. Soils are trampled, particularly in areas around ring feeders. Trafficking to feed stock can also cause soil compaction.

Runoff from compacted soil caused by out-wintering of sheep

Cultivation is often necessary to improve soil structure in badly damaged fields following out-wintering of stock.

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Environment Agency thinksoils factors that influence erosion and runoff / land use / livestock

Harvesting forage crops

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Maize and other forage crops are often harvested when soils are wet and vulnerable to compaction during autumn (and winter). Compacted maize stubble is a common cause of field runoff during winter.

land use / livestock

These crops are also at risk to runoff during heavy rainfall in early summer when there is a lack of crop cover and the soil becomes capped (or due to runoff along compacted tramlines).


land use /

Wheel ruts caused during harvesting of maize

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Spreading slurry and manure during winter Soil structure is at risk of being damaged when slurry and heavy loads of manure are spread in the winter onto wet soft soils. Where soils are compacted slurry can run off the soil surface during rainfall.

Slurry spreading on wet soil is at high risk of being washed off

Environment Agency thinksoils factors that influence erosion and runoff / land use / livestock

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land use / Runoff from potatoes grown on long steep slopes

land use / root crops and vegetables Spring land work Growing root crops and vegetables often involves deep cultivation, stone removal and clod separation, bed forming, and use of plastic or fleece. These operations are carried out in spring when subsoils can still be too wet and vulnerable to compaction by heavy machinery.

Fine tilths (e.g. for carrots, onions and sugar beet) on sandy and peaty soils are also vulnerable to wind erosion

In steep fields, rows and beds are formed up and down the slope because harvesting equipment can not operate across the slope. Rows and beds channel water downhill increasing the risk of rapid water runoff.

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Environment Agency thinksoils factors that influence erosion and runoff / land use / root crops and vegetables

Headlands, tramlines and tracks Potatoes and vegetables involve many vehicle movements to spray crops and for irrigation. Headlands, tramlines and tracks are most at risk of becoming compacted and causing runoff.

land use / root crops and vegetables

Compacted headlands and wheelings causing runoff and soil erosion

Lifting and transporting heavy loads of potatoes causing soil compaction


land use /

Autumn harvesting Where root crops and vegetables are harvested in autumn and winter (when soils are wet and soft) there is a high risk of causing compaction and runoff. Transporting heavy loads of root crops is a major hazard to the soil in wet conditions where wheel ruts result in deep compaction and subsequent channelling of runoff.

Environment Agency thinksoils factors that influence erosion and runoff / land use / root crops and vegetables

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